1. Field of the Invention
The invention resides in the field of continuous processes for rapidly making supported ion exchange membranes from polymerizable liquid precursors having no non-polymerizable diluents (other than water) which have boiling points of less than about 185.degree. C. and from quasi-continuous, foraminous, solid substrates such as fabrics, paper and microporous diaphragms and to the products of such processes.
2. Description of the Prior Art
During the last four decades several million square feet of ion exchange membrane have been made from freely flowing solutions of functional or functionizable monomers, crosslinking monomers and low molecular weight, non-polymerizing diluents for said monomers. (See e.g. U.S. Pat. No. 2,730,786). The diluents are also characterized by being good swelling agents for the polymers eventually resulting from such monomers and by being readily leached out of such polymers. Almost all of such membranes have been reinforced with substrates such as woven or non-woven fabrics, paper or microporous diaphragms. The process generally comprised interleaving the substrates and glass plates in a pool of such solution in shallow trays, removing excess solution, polymerizing the monomers by heating the array of plates and substrates for several hours and subsequently disassembling the array to retrieve the polymer impregnated substrates. The glass plates were generally recycled. In some few cases the polymer impregnated substrates were useful as ion exchange membranes after simple leaching in water (or other solvents and then water). Generally however functional monomers had first to be converted to suitable ion exchange groups (e.g. polyvinyl benzyl chloride-co-divinyl benzene to poly-(N-benzyl-N,N,N-trimethyl ammonium chloride)co-divinyl benzene) or ion exchange groups had to be added to functionizable moieties (e.g. sulfonic acid groups to polystyrene-co-divinyl benzene).
Such process, as is, is inherently labor intensive and difficult to automate or robotize. The non-polymerizing organic diluents generally used in the above mentioned solutions of monomers have had boiling points of less than about 185.degree. C., e.g. methanol (65.degree. C.), isopropanol (82.degree. C.), dioxane (102.degree. C.), isobutanol (108.degree. C.), toluene (111.degree. C.), diethyl benzene (183.degree. C). As a result substantial local air pollution ensued during interleaving the substrates and glass plates in a pool of such solution and during subsequent disassembly of the array to retrieve the polymer impregnated substrates. Finally the diluents had to be leached out with water (or other solvents and then water) as noted above thereby posing a disposal or recovery and recycle problem. Further the high volatility of some such traditional diluents prevents carrying out the polymerization rapidly, e.g. in 10 to 40 minutes because the high energy release during such rapid polymerization results in boiling of the diluent, forming bubbles in the polymer and/or forcing polymerizing liquid out of the substrates. Hence the use of low boiling, environmentally unfriendly diluents had rendered uneconomic until now continuous machinery for making ion exchange membranes since the production rate of such machines is uneconomically low if the polymerization time must be several hours. For example U.S. Pat. No. 4,231,855 describes cation exchange membranes prepared from 2-sulfoethyl methacrylate, ethylene glycol dimethacrylate and benzoyl peroxide. The non-polymerizable diluent was isobutyl alcohol (b.p. 108.degree. C.). The polymerizable mix was poured into a tray in which were laid in alternating fashion glass plates and cloth substrates. The resulting array was heated at 80.degree. C. for 17 hours. It is obvious that a continuous machine having, for example, a heating zone 48 feet in length would have a production rate under such circumstances of only 0.047 linear feet per minute (i.e. 68 linear feet per 24 hours).
The above mentioned problems are overcome according to the present invention which inter alia provides a process for rapidly manufacturing reinforced ion exchange membranes from:
(I) a liquid characterized by having a viscosity less than about 10 poise at shear rates in excess of about 1 cm/sec and comprising at least in part components polymerizable to such membranes and by comprising no non-polymerizable solvents other than water which have a boiling point of less than about 185.degree. C.; and from:
(II) at least one quasi-continuous, pliable, support substrate having a multiplicity of apertures extending through the substrate from one face thereof to the other face thereof, said substrate further characterized by being insoluble in the above mentioned liquid at temperatures and times prevailing during the process.
The process comprises:
(a) forming a continuous sandwich of the substrate (or a pack of substrates) by juxtaposing on each face of the substrate pliable film extending beyond the side edges of the substrate, the film characterized by not being swollen by or permeable to the above mentioned liquid at the temperatures and times prevailing during the process;
(b) passing the sandwich of substrate and film in a more or less vertical direction downwardly between a pair of horizontal squeeze rolls and joining the side edges of the film before passing the sandwich through the squeeze rolls;
(c) introducing the above mentioned liquid into the pocket formed in the sandwich above the squeeze rolls;
(d) pulling the sandwich and liquid between the squeeze rolls thereby filling the apertures in the substrate with the liquid;
(e) thereafter rapidly polymerizing the polymerizable components in the liquid resulting in the formation of a reinforced polymer sheet.
Preferably the support substrate is a woven or non-woven fabric although microporous diaphragms can equally well be used, e.g. microporous polyvinyl chloride or polyacrylonitrile diaphragms.
Preferably the components in the above mentioned liquid which are polymerizable comprise polymerizable electrolytes such as vinyl sulfonic acid, styrene sulfonic acid, 2-sulfoethyl methacrylate, 3-sulfopropyl acrylate, 3-sulfopropyl methacrylate, 2-methyl-2[(1-oxo-2-propenyl)amino]-1-propane sulfonic acid, 2-methyl-2-[(1-oxo-2-methyl-2-propenyl)amino]-1-propane sulfonic acid, vinyl benzyl trialkyl ammonium hydroxide, methacrylatoethyl-trialkyl ammonium hydroxide, methacrylamido-propyl trialkyl ammonium hydroxide, N-alkyl-2-methyl-5-vinyl pyridinium hydroxide, N-methyl-2-vinyl pyridinium hydroxide, N-methyl-4-vinyl pyridinium hydroxide, vinyl benzyl dialkyl sulfonium hydroxide, N,N-dialkyl amino ethyl methacrylamide, N,N-dialkyl amino ethyl acrylate, N,N-dialkyl amino ethyl methacrylate, 3-dialkyl amino neopentyl acrylate, 3-N,N-dialkyl aminopropyl methacrylamide and their salts.
Generally the above mentioned alkyl groups will be methyl though other alkyl or mixed alkyl groups can be used, for example hydroxy ethyl. Particularly useful among the salts of such polymerizable electrolytes are the McKee type salts in which the counterion is a low molecular weight non-polymerizable organic ion. Examples of such McKee type polymerizable electrolytes include:
vinyl benzyl trimethyl ammonium paratoluene sulfonate PA0 benzyl dimethyl sulfonium styrene sulfonate PA0 vinyl benzyl dimethyl sulfonium benzene sulfonate PA0 benzyl trimethyl ammonium parastyrene sulfonate PA0 tetraethyl ammonium parastyrene sulfonate. PA0 vinyl benzylhalide and N,N-dialkyl amino ethyl methacrylamide,3-N,N-dialkyl amino propyl methacrylamide, N,N-dialkyl amino ethyl methacrylate, N,N-dialkyl amino ethyl acrylate, vinyl benzyl dimethyl amine, vinyl benzyl methyl sulfide, 3-dimethyl amino neopentyl acrylate, 2-methyl-5-vinyl pyridine, 2-vinyl pyridine or 4-vinyl pyridine PA0 vinyl benzyl halide and dimethyl ethanol amine or triethanol amine PA0 alkyl paratoluene sulfonates or alkyl benzene sulfonates and any of the above mentioned polymerizable tertiary amines or secondary sulfides.
The use of such polymerizable electrolytes as at least one component of the above mentioned liquid avoids (after the above mentioned polymerization step) a chemical reaction step i.e. a step in which further covalent bands are formed in order to introduce ion-exchange groups (e.g. by sulfonation) or to convert functionizable moieties to ion exchange groups (e.g. by alkylating tertiary amine groups or hydrolyzing sulfonic ester groups). Such chemical reaction steps are generally costly and generally produce waste products which are difficult to dispose of.
Alternatively the components in the above mentioned liquid which are polymerizable comprise species which spontaneously form ion exchange groups during polymerization. Examples include:
Preferably any non-polymerizable diluents other than water are not only high boiling (i.e. have a boiling point greater than about 185.degree. C.) but have a low level of toxicity to humans, are soluble in water and are biodegradable. Preferred diluents other than water meeting such requirements include:
______________________________________ Dielectric Solubility B.P..degree.C. Constant Parameter ______________________________________ dimethyl sulfoxide 189 47 11.8 formamide 210 109 16.2 1,3 propylene glycol 214 35 12.1 1,3 butane diol 208 10.8 dipropylene glycol 232 9.1 1,2-propylene glycol 188 32 11.6 propylene carbonate 242 65 11.4 gamma-butyrolactone 204 39 11.6 1-methyl-2-pyrrolidinone 202 10.3 sulfolane 287 43 11.4 glycerin 290 43 13.0 1,2,3-butane triol 264 11.5 1,2,6-hexane triol &gt;185.degree. 1,2,4-butane triol &gt;185.degree. ______________________________________
(The solubility parameters listed above are calculated on an internally self-consistent basis and differ slightly from values reported in the literature).
The components in the above mentioned liquid which are polymerizable generally also comprise monomers capable of crosslinking the resulting polymers. Preferred crosslinkable monomers include divinyl benzene, ethylene glycol dimethacrylate, bisphenol-A-dimethacrylate, 1,3 butane diol diacrylate, 1,4 butane diol diacrylate, 1,3 butane diol dimethacrylate, 1,4 butane diol dimethacrylate, 1,4-cyclo hexane dimethacrylate, diethylene glycol diacrylate, diethylene glycol dimethacrylate, 2,2-dimethyl propane diol dimethacrylate, para phenylene diacrylate, N,N'-dimethylene bisacrylamide, N,N'-methylene bisacrylamide and N,N' methylene bis methacrylamide.